The soft X-ray component seen at the peak of the hard state during the
outburst of SWIFT J1753.5-0127 is clearly from the disc. While the
system parameters are rather poorly known, the radii derived close to
the peak are \(\sim 15~R_g\) , corresponding to \(\sim 45~R_g\)  after a
colour temperature correction. This is consistent with the disc being
recessed back from the last stable orbit, as required for the
truncated disc/hot inner flow interpretation of the hard state. There
are two independent lines of support for this number. The first comes
from the fact that the hard X-ray spectrum is softer close to the
peak, clearly consistent with the disc providing an increasing source
of seed photons to Compton cool the hot X-ray plasma. Since most of
the gravitational potential energy to power this corona is
concentrated within \(20 R_g\)  then it seems most likely that the disc
extends down to similarly small radii. This means that there can also
be irradiation of the inner disc by the hard X-ray emission, leading
to reflection and thermalisation of the incident flux. We see evidence
for both these processes in the spectrum, with the extra flux from
irradiation increasing the inferred disc radius from the value of
\(\sim 11~R_g\)  inferred from the simple (unirradiated) disc fits to the
X-ray spectrum, to \(15~R_g\)  (i,e, from 30 to \(45R_g\)  after colour
temperature correction). The second is from low frequency QPO which
is at its maximum of \(\sim 0.8\)  Hz in these data. This implies an
outer radius of \(\sim 20-30~R_g\)  assuming that the QPO is produced by
Lense-Thirring (vertical) precession of the hot flow around a
\(10~M_\odot \)  black hole [1]}.
